1. Field of the Invention
The present invention relates generally to an LSI device, and more particularly, to an LSI device capable of adjusting the output impedance so as to match the characteristic impedance of a transmission line irrespective of the variance of use environments such as temperatures and power supply voltages.
2. Description of the Related Arts
The LSI device outputs a processed signal from a data output circuit in the LSI device to a transmission line. In this case, it is desirable that output impedance of the data output circuit is matched with characteristic impedance of the transmission line. This is because, if the data output circuit is matched with the impedance of the transmission line, a reflection of a signal at a connection point of the both disappears, and it is possible to prevent damping of the signal to be transmitted to the transmission line.
For this reason, in the conventional LSI device, a simulation of the connection of the data output circuit with the transmission line is made to determine an output transistor size of the data output circuit, or the data output circuit is connected actually with the transmission line to observe a signal waveform, and the transistor size of the data output circuit is trimmed, so that the output impedance is matched to the characteristic impedance of the transmission line.
However, if the output transistor size of the data output circuit is fixed by trimming, when a manufacture process fluctuates, or when fluctuations such as temperatures, power supply voltages, characteristic impedance of the transmission line, or the like generate in the case where the LSI device is used, the output impedance of the data output circuit is not matched with the characteristic impedance of the transmission line.
Furthermore, after the LSI device is manufactured, in the case where a user""s use conditions in use temperatures, power supply voltages, characteristic impedance of the transmission line, or the like are changed, it is impossible to agree the output impedance with the user""s use conditions by changing the output transistor size of the data output circuit.
It is therefore the an object of the present invention to provide an LSI device which can be adjusted so that output impedance of a data output circuit is matched with characteristic impedance of a transmission line even if manufacture processes, use temperatures, power supply voltages, characteristic impedance of the transmission line, or the like fluctuates or is changed.
In order to achieve the above object, according to an aspect of the present invention there is provided an LSI device having an output terminal outputting a data, comprising a data output circuit connected to the output terminal and capable of adjusting an output impedance thereof; and an adjustment circuit which detects a transient voltage at the output terminal when an output logic of the data output circuit is switched in a condition that a transmission line not terminated by a terminating resistor is connected to the output terminal, compares the transient voltage with a reference voltage, and adjusts the output impedance of the data output circuit so as to match a characteristic impedance of the transmission line.
According to the present invention, as the transient voltage of the output terminal connected to the transmission line is detected, and the output impedance of the data output circuit is adjusted, it is possible to adjust so that output impedance of the data output circuit is matched with characteristic impedance of the transmission line even if manufacture processes, use temperatures, power supply voltages, characteristic impedance of the transmission line, or the like fluctuates or is changed.
In order to achieve the above object, according to another aspect of the present invention there is provided an LSI device having an output terminal outputting a data, a dummy output terminal outputting a dummy data, and a dummy input terminal which accepts the dummy data through a dummy transmission line, the LSI device comprising a data output circuit connected to the output terminal and capable of adjusting an output impedance thereof; a dummy output circuit connected to the dummy output terminal and having a configuration equivalent to that of the data output circuit; and an adjustment circuit which detects a transient voltage at the dummy output terminal when an output logic of the dummy data output circuit is switched in a condition that a dummy transmission line is connected between the dummy output terminal and the dummy input terminal, compares the transient voltage with a reference voltage, and adjusts the output impedance of the data output circuit and of the dummy output circuit so as to match a characteristic impedance of the dummy transmission line.
According to the present invention, as the transient voltage of the dummy output terminal connected to the dummy transmission line is detected to adjust the output impedance of the dummy output circuit and data output circuit, even if manufacture processes, use temperatures, power supply voltages, characteristic impedance of the transmission line, or the like fluctuates or is changed, the output impedance of the data output circuit can be adjusted so as to match the characteristic impedance of the transmission line.
Preferably, the transient voltage is detected at the timing when a logic of a signal fed to the dummy input terminal is switched, and the transient voltage is a voltage substantially intermediate of the amplitude of the signal.
According to the present invention, as the transient voltage can be detected at the timing that the logic of the signal to be input into the dummy input terminal is switched, it is possible to readily detect a substantially intermediate voltage of the signal amplitude.